Occasionally when resending welcome emails, instead of the email sending, the user will receive a 404 error.

This is a very rare bug but it does happen. The current workaround, if this continues to happen to you, is to simply dump that user and re-set up their account. To do this, make sure that you change their name and email in the active account, then deactivate it. Once the user is changed and deactivated, re-create that user's account with or without a welcome email.